Your client's permanent establishment & his citizenship/US Green Card determines his US taxation and not just the fact that he has a US employer, assuming Google USA is his employer.

Please also note that Google is generally very accurate in multi jurisdictional income reporting for its employees. They allocate multi-year income (equity, bonuses, etc) based on each countries rules and report and withhold taxes as required. He may have year-end wage and withholding statements for each of the jurisdictions where his income was sourced. You should request his equity statements, if you do not already have them, so that you can make sure you do not double count any income (assuming you are preparing the US return).